As the number of electronic elements contained on semiconductor integrated circuits continues to increase, the problems of reducing and eliminating defects in the elements becomes more difficult. To achieve higher population capacities, circuit designers strive to reduce the size of the individual elements to maximize available die real estate. The reduced size, however, makes these elements increasingly susceptible to defects caused by material impurities during fabrication. These defects can be identified upon completion of the integrated circuit fabrication by testing procedures, either at the semiconductor chip level or after complete packaging. Scrapping or discarding defective circuits is economically undesirable, particularly if only a small number of elements are actually defective.
Relying on zero defects in the fabrication of integrated circuits is an unrealistic option. To reduce the amount of semiconductor scrap, therefore, redundant elements are provided on the circuit. If a primary element is determined to be defective, a redundant element can be substituted for the defective element. Substantial reductions in scrap can be achieved by using redundant elements.
One type of integrated circuit device which uses redundant elements is electronic memory. Typical memory circuits comprise millions of equivalent memory cells arranged in addressable rows and columns. By providing redundant elements, either as rows or columns, defective primary rows or columns can be replaced. Thus, using redundant elements reduces scrap without substantially increasing the cost of the memory circuit.
Because the individual primary elements of a memory are separately addressable, replacing a defective element typically comprises blowing fuse-type circuits to `program` a redundant element to respond to the address of the defective element. This process is very effective for permanently replacing defective primary elements. A problem with this process is the possibility of replacing a defective primary element with a defective redundant element. The possibility of having a defective redundant element increases as the number of redundant elements on an integrated circuit increases. Because the process of replacing defective elements is a permanent solution, if a defective redundant element is used, the circuit must be scrapped.
The number of redundant elements provided on a circuit usually exceeds the number of redundant elements needed to `repair` a defective chip, therefore it would be desirable to replace the defective redundant element with another available redundant element.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a circuit and method for cancelling and replacing defective redundant electronic elements on an integrated circuit.